You need to create the lookup, internal or external, as a field but once that field exists you can use it in other calculated fields.

The only word of caution is that lookups (external) in general (not just in Monarch) need to be treated with some care to ensure that what you ask a system to do will fit well with the availability of the data for the lookup. So, if you are creating a lookup to a static table of file there should be no problems of much importance but if you are creating a lookup that relies an variable data, process by process, check the results carefully to ensure that the lookup is making use of the most current data you need it to use.